1. Field of the Invention
The present invention relates to a communication method, particularly to a visible light communication method.
2. Description of the Related Art
Visible light communication (VLC) generally refers to short distance communication using a visible light wavelength of 380˜780 nm. Compared with ordinary light sources, light emitting diodes (LEDs) feature high lighting efficiency, high modulation speed, long life, etc. As a result, LEDs are viewed as main stream illumination sources in next generation. VLC can transmit data using the existing illumination and display equipments; hence it is fascinating for its additional economical value. Moreover, high-speed modulation capability of LEDs also urges VLC into a studying issue which is highly concerned.
According to the CIE1931 model, a specific color can be generated by mixing a specific intensity ratio among R, G and B lighting colors. Let the total power of the R, G, and B sources be fixed under a specific value. The relative intensities of the R, G, and B sources are then normalized by this total power. The final output color composed by the three colors is defined on a 2-dimension plane. By controlling the relative intensities among the three color sources, any color can ideally be obtained. Dimming refers to modulating the brightness of an illumination source. Dimming technique is especially beneficial for the illumination and display systems for it can provide high illumination/display qualities, such as color uniformity and color precision. Moreover, dimming can also be used to more efficiently using energy. Hence, high-speed and precise dimming methods are prosperously pursued.
Analog modulation and pulse-width modulation (PWM) are two common dimming methods. Analog modulation refers to modulating the brightness of a LED by adjusting the injection current of the LED. By moderately adjusting the relative intensities of the three color LEDs, arbitrary color can be obtained according to the CIE1931 model. However, analog modulation suffers from low illumination/display quality for its gradual color transition appears while any two specific colors are switched from each other. Consequently, dimming by analog modulation can not satisfy the requirement for the high-quality illumination and display systems. On the other hand, according the human sensation mechanisms, the color and brightness perceived by a human eye are determined by the total received energy of light within a specific interval. Based on this principle, PWM divides the light irradiated by each LED into several discrete pulses with different pulse widths. Hence, the total light energy received by a human eye can be adjusted by moderately controlling the widths and the amount of the pulses within the sensation interval (integration time of a human eye). Dimming using PWM can reduce color shift due to analog variations. Dimming using PWM can also use energy more efficiently because light is radiated only while the discrete pulses are sent.
In the applications of VLC, a LED itself is an incoherent source. Thus, only the amplitude of light can be modulated for transmission signals. As mentioned above, controlling the color and brightness of light also relates to modulate the amplitude of light. It is difficult for a receiver to distinguish a signal from its receiving power level because the power of the discrete pulses should not be changed or the brightness and color of the output light may be varied. The receiver will always receive the power level determined by the dimming requirement. Variable pulse position modulation (VPPM), PWM-based PPM, etc. are proposed to mitigate this dilemma to realize simultaneous dimming and signal transmission. Nevertheless, it is impractical to realize high-speed VLC using the abovementioned modulation formats. The reasons are described as the following: (1) Transmission signals cannot most efficiently use the bandwidth of a LED because in a PPM and PWM system, data can only be loaded while the PWM pulses are sent. (2) The jitter and multipath will decrease the signal performance. As a result, the signal speed cannot be too fast (the lower the data speed is, the lower the demands for precision of locating data time are) lest bit error occur. (3) The data allocation for different transmission channels cannot be optimized.
To overcome the abovementioned problems, the present invention provides a VLC method, so as to solve the afore-mentioned problems of the prior art.